The invention relates to a honeycomb, particularly a catalytic converter substrate, pursuant to the generic part of Claim 1 and a process for its manufacture.
A honeycomb of this type is known from DE 27 33 640, which consists of an alternating arrangement of corrugated and plane foil layers. The flow ducts, which display a sinusoidal or triangular cross-section, are, however, unfavourable in terms of the catalytic function, as the gussets of the ducts are virtually ineffective, particularly in the case of laminar flow. In addition, the properties of the honeycomb under fluctuating temperature conditions are unfavourable because the large-area soldering of the plane and corrugated foils results in a very stiff honeycomb structure. As a result, however, local and temporal temperature fluctuations cannot be adequately balanced out, as a result of which the geometry of the flow ducts is subject to irreversible changes and cracks can occur in the cell walls, this being intensified by vibratory stresses on the honeycomb. The service life of honeycombs of this kind is thus in need of improvement.
In order to increase the stability of the honeycomb, it is moreover known from EP 0 245 738 that rigid bearing walls extending into the honeycomb are provided. The manufacture of honeycombs of this kind is, however, relatively complex, as the foil layers have to be cut through for this purpose. In addition, the fastening of the thin foils to the comparatively thick, rigid bearing walls presents a problem.
The object of the invention is to create a honeycomb that displays sufficient stability with high resistance to thermal shocks, that permits the most favourable possible design of the flow ducts in terms of flow and that is simple and inexpensive to manufacture.
According to the invention, this object is solved by a honeycomb with the features of Claim 1. Due to the fact that the dimensions of the stiffening elements, which run essentially parallel to the foil layers, transverse to their longitudinal direction, is small compared to the dimensions of the honeycomb structure in this direction, mass transport within the flow ducts, and thus also the effective reaction cross-section of the ducts, is virtually not reduced. Unfavourable cross-section geometries resulting from the formation of gussets thus virtually do not occur at all, or only in areas of small volume. In addition, owing to their orientation parallel to the foil layers, the stiffening elements can easily be incorporated into the honeycomb structure during its manufacture. A certain degree of stiffening is already achieved by the minimum distance between adjacent foil layers being limited in the case of structured foils, meaning that foil structures, for example, are supported by the stiffening elements. In particular, the stiffening elements can prevent elongation of the honeycomb structure in a direction perpendicular to the flow ducts or foil profiles, which would lead to the undesirable formation of spaces between foil layers of different elongation and thus to unfavourable vibratory stresses in the honeycomb structure. Moreover, as a result of the stiffening elements introduced, the honeycomb according to the invention can be made up of foils with virtually any desired structure or orientation, as it is no longer necessary to fasten the foil layers to each other.
The stiffened areas of the honeycomb structure can have punctiform or locally isolated dimensions or, in the case of stiffening elements of corresponding length, they can form stiffening zones. In all cases, the stiffening elements according to the invention locally fix the foils relative to each other more strongly, thus producing larger areas of the honeycomb structure displaying high flexibility.
In order to achieve sufficient stabilisation of the honeycomb structure, it suffices in itself for the length of the stiffening elements, regardless of the direction in which they extend, to be equal to or greater than the transverse dimension of a duct in one direction, e.g. height or width, and for them to bridge a flow duct, for example, i.e. to act on opposite walls of a duct or the housing. Also, the stiffening elements can extend only over several duct diameters transverse to the longitudinal direction of the duct, e.g. 5 to 10 duct diameters, or over the entire width of the honeycomb. In the case of non-isometric or non-isogonal ducts, the stiffening elements can also extend over only part of the duct cross-section amounting to a multiple of the duct dimension in the cross-sectional direction of smaller size, e.g. twice this dimension or more.
Advantageously, the dimension of the stiffening elements transverse to their longitudinal dimension is small compared to the dimensions of the flow ducts in this direction, e.g. in the range of {fraction (1/10)} to {fraction (1/50)} of the dimension of the flow ducts in this direction or less, without being limited to these values. The transverse dimension of the stiffening elements can be {fraction (1/100)} to {fraction (1/1000)} or less of the duct length, for example, if these run transverse or at an angle to the ducts. Accordingly, when using the same material, the width of the stiffening elements can be just 0.5 to 10 times, preferably 1 to 5 times, the thickness of the foils making up the honeycomb structure, without being limited to these values.
If, for example, a honeycomb is available which has a flow duct length of 100 mm and a flow duct diameter of 1 mm, strip-like stiffening elements with a width of several millimeters and/or stiffening wires with a diameter of several hundredths to several tenths of a millimeter arranged transverse to the flow ducts can be provided. If the stiffening elements are arranged in the longitudinal direction of the ducts, their width can be in the range of 0.01 to 0.5 mm, preferably 0.003 to 0.2 mm. It goes without saying that, given corresponding honeycombs with larger duct diameters, which can easily also be in the region of approx. 1 cm or more for corresponding applications, the stiffening elements can display correspondingly larger diameters or widths.
It is also possible for several stiffening elements to be assigned to one foil or one pair of foils or several adjacent foils.
The stiffening elements preferably extend over the entire honeycomb structure in their longitudinal direction.
The stiffening elements are advantageously designed to be elastically deformable under operating conditions, perpendicular to their longitudinal direction, particularly in the direction of the flow ducts.
The stiffening elements can run between adjacent foil layers, although they can also pass through profiled foils or be woven into plane foils, and/or connect adjacent foils to each other.
In their longitudinal direction, the stiffening elements are advantageously connected to the foil layers and/or the housing in a manner capable of absorbing tensile forces, e.g. by means of suitable jointing techniques, such as welded connections, positive, frictional and/or material connections. However, connection of the stiffening elements to the foil layers, in particular, can also be achieved by coating with a ceramic material required to produce a catalytic coating.
In order to achieve frictional connection of the stiffening elements to the foil layers, the stiffening elements can be woven into the foil layers, particularly connecting two adjacent foil layers in the process, or be clamped in corresponding folds in the foils. Areas of the foils can be notched out to this end, or the stiffening elements can be inserted into the folds of connecting webs located at the face ends of the foils. Correspondingly, the structured areas, such as the foil corrugations, can also be provided with notched tabs or projections running in the longitudinal direction of the ducts, these being arranged one behind the other, possibly at an offset height, and forming a lead-through for wires or the like running parallel to the flow ducts.
However, an increase in the dimensional stability of the honeycomb is already achieved if the stiffening elements loosely support the foils or are loosely passed through one or more foils, e.g. by providing an appropriate profile.
Particularly if they are located at the level of a foil layer or between the foil layers, the stiffening elements can also be connected to each other by way of additional stiffening or connecting struts, which can run essentially parallel to the foil layers and/or perpendicular to them. In this way, extended systems of stiffening elements can be constructed, which can extend in two or three dimensions over relatively large areas of the honeycomb or the entire honeycomb. Correspondingly, in order to stiffen the honeycomb structure, expanded-metal layers or wire mesh can also be inserted between the foil layers, these particularly being inserted into indentations of foil layer profiles and possibly secured there in a manner preventing movement.
Advantageously, the stiffening elements are connected to the foil layers under axial pretension. This makes it possible not only to increase the stiffness of the honeycomb, but also to calibrate the geometry of the flow ducts or the dimensions of the honeycomb. In this context, the stiffening elements can be connected both to the housing of the honeycomb and to existing partition walls, these being designed as rigid bearing walls or as elastically deformable partition walls composed, for example, of fold areas of the foil layers. The fold areas can be of U, V, W or Z-shaped design, without limitation, in which context individual or several legs of the fold are joined together in order to construct the wall. The folded design of the partition walls means that they are flexible and, at the same time, that they expand under compression, this resulting in good temperature shock resistance.
If the stiffening elements running through the honeycomb structure are pretensioned, the pretensioned area of the corresponding foils can be grouped in sections. This makes it possible, for example, to provide block-type areas of high pretension and thus high stiffness within the honeycomb structure that are separated by areas of low pretension and thus increased deformability.
This kind of design with pretensioned areas within the honeycomb structure can be produced by the fastening elements on the foil layers for fastening the stiffening elements only being provided in some areas. Thus, for example, the connecting webs of foil strips folded in zigzag fashion can be removed in some areas of the lateral edge zones of the honeycomb, thus providing a zone of increased extensibility adjacent to the housing and producing a honeycomb with particularly favourable mechanical properties.
The cross-sectional geometry of the flow ducts can be adjusted by pretensioning stiffening elements fastened to the foil layers.
According to another advantageous configuration, the stiffening elements can be formed from partial sections of the foil layers.
This is particularly the case if the honeycomb is formed by a foil strip folded in zigzag fashion, where the individual foil layers are connected to each other by web-like connections in the area of the folds. In this context, the connecting webs in the area of the folds can be produced by way of punched tabs, where the fold line of the adjacent folded sections of the foil strip runs through the punched tab. The punched tab can be designed in such a way that a web running along the fold line remains, meaning that the wall areas of a flow duct that are opposite each other along the fold line can be connected to each other. In order to permit corrugation of the foil strip, the web running through the cross-section of a flow duct can be shortened by bending or folding appropriately in its longitudinal direction.
In addition, or as an alternative, to the configurations described, it is also possible to provide stiffening elements which are designed as inserts that can be inserted into the face ends of the ducts. The inserts, the outside contours of which can be adapted to the cross-sectional geometry of the ducts, prevent adjacent foil layers from sliding into each other, without substantially affecting the flow cross-section of the ducts. The inserts can be designed in such a way that they display areas that protrude from the face ends of the honeycomb when inserted and act as flow-guiding devices. These areas, which can be integrally moulded, can enable lateral inflow into the inlet area of the honeycomb and/or be arranged at an angle to the longitudinal direction of the honeycomb.
The inserts can be designed as separate components and advantageously extend over the width, possibly also over the height, of several ducts, or over the entire width and/or height of the honeycomb structure. Stiffened areas of the honeycomb structure can alternate with areas of increased extensibility in this way. By varying the arrangement of inserts at both face ends of the honeycomb, it is possible, for example, to obtain twistable honeycombs, which may be advantageous for certain fields of application. If the inserts extend over several ducts, they can be arranged both parallel and perpendicular or at an angle to the foil layers.
The inserts can also be integrally moulded to the foil layers and produced, for example, by appropriate folding of foil sections. The inserts can be designed to suit the requirements by shaping the foil ends or by punching.
Particularly if the stiffening elements are designed as inserts, the flow cross-sections can easily be varied over the length of the flow ducts. For instance, the inserts can be profiled in such a way that the flow ducts have a smaller diameter in the turbulent inlet area of the ducts than in the duct areas with laminar flow inside the honeycomb. In this context, the inlet area is advantageously divided into a large number of flow ducts, so that the total of the flow cross-sections of the ducts in the inlet area is roughly equal to the flow cross-section of the duct in the middle area of the honeycomb.
As an alternative, or in addition, to the configurations described above, the stiffening elements can also be designed as webs running along the flow ducts. In this context, the webs display a width which is substantially smaller than that of any profiles provided for producing the honeycomb structure, e.g. one-quarter or one-eighth of the same, or less. In the case of webs consisting of two side walls, both fold legs can, in particular, contact each other, advantageously over virtually the entire height, or only be such a distance apart from each other that the respective coating compound used does not penetrate the space between the legs.
The webs can extend over the entire height of the ducts or, advantageously, only over part of the same, so that gas exchange between the constituent ducts is possible. The webs can also display notched tabs, by means of which adjacent foil layers are supported or which serve to increase the catalytically active surface area. The notched tabs or the webs themselves can be used to fasten or support further stiffening elements, such as wires running transverse to them. The webs can, in particular, be designed as fold webs of the foil layers, there being beaded areas at the ends or in the middle area of the fold webs for additional stabilisation, these counteracting any spreading of the fold webs. Wires or the like can additionally be inserted in the fold webs. Fold webs may, for instance, protrude from the foil layers in a direction inclined or perpendicular to the major plane of the foil layers or at least substantially in parallel to the foil layers, which might be achieved for instance by folding the webs in a lateral direction generating double or multiple folded foil layer sections heving for instance 3 to 10 or even more folds.
In the case of structured foil layers where partial areas of one and the same foil layer are in punctiform or linear contact, the honeycomb can be stabilised by establishing punctiform or linear connections between contacting foil areas in the areas of contact. This has an influence on the longitudinal expansion characteristics of a single foil layer, where the connections can be at a distance from the top side of the structured foil layer, forming expansion legs. The foil layer as a whole thus acts as a stiffening element capable of absorbing tensile and/or compressive forces, which is produced by joining individual preformed sections of the foil layers and extends over individual or several flow ducts or results in layer doubling. Correspondingly, individual sections of the foil layers, e.g. in web form, can be notched out and joined to each other or with the foil layers in a manner capable of absorbing tensile forces in order to produce stiffening elements.
The joints within an individual foil layer can be produced by any desired jointing techniques, e.g. by spot welding, and, in particular, positive connections can be produced by means of punched and folded foil areas, the face ends of which engage an adjacent flow duct or apertures correspondingly provided for this purpose, or which are non-positively connected to the wall of a flow duct.
In addition to the stiffening elements inserted according to the invention, it is also possible to provide stiffening elements of a wide variety of configurations which extend in a perpendicular direction relative to the foil layers and connect two or more foil layers. Perpendicular direction is generally intended to mean a direction possessing a perpendicular direction component and including an angled course, e.g. at an angle of 45xc2x0 relative to the foil layers.
The perpendicular stiffening elements can be designed as rigid bearing walls, although they are preferably of elastically deformable design, where one-dimensional stiffening elements can be provided, in the form of wires, strips, interconnected foil folds or the like, or two-dimensional elements in the form of deformable outer or partition walls, which particularly consist of folded sections of the foil layers. The stiffening elements running parallel to the foil layers can be fastened to the stiffening elements running vertical to the foil layers in a manner capable of absorbing tensile forces, or they can be loosely passed through or by these.
According to another advantageous configuration, the stiffening elements are located upstream, or in the inlet area, of the flow ducts, i.e. in the area of turbulent flow. The stiffening elements, which can particularly extend transverse to the flow ducts, thus form additional catalytically active surfaces at the same time. In addition, or as an alternative, projecting areas with catalytically active surfaces can also be located in the catalytically particularly effective inlet area by other measures. In particular, the stiffening elements in the inlet area, which can also be designed as strips or wires, can have a larger diameter than in the area of laminar flow. The inlet area reinforced with stiffening elements can also display foil layer sections with free ends which permit lateral inflow of a fluid on one or more sides.
If the stiffening elements are located upstream of the face ends of the flow ducts, it has proven advantageous for the distance between the outer edge of the stiffening elements facing away from the flow ducts and the face ends of the flow ducts to be in the range of 0.1 to 3 times the diameter of the ducts. If the foils are connected to each other by fold webs lying on fold lines located upstream of the face ends of the ducts, the same applies to the distance between the fold lines and the face edge of the inlet or outlet apertures of the ducts. This applies regardless of whether or not stiffening elements are located in the fold areas.
In order to improve the flow conditions, particularly in the event of angular inflow into the ducts, the duct ends can be of scoop-like design. It is also possible to provide window-like foil folds at the duct ends in order to enlarge the inlet areas with turbulent flow.
It goes without saying that the honeycomb according to the invention can be constructed not only from a profiled foil strip laid in zigzag fashion, but also from individual profiled foils, between which non-profiled foils may possibly also be arranged. In particular, individual foil layers can also be arranged one above the other in such a way that the flow ducts are produced by the profiles of opposite foil layers. The stiffening elements can also be provided within the honeycomb at a distance from the face ends of the same.